1. Field of the Invention
The present invention concerns an omnidirectional antenna. This antenna can be applied especially to the transmission of radio or television broadcasting signals in the decimetric wavebands (the so-called microwave band) where it will be seen that it gives particularly worthwhile advantages.
However, the invention is limited neither to this application nor to this band of frequencies, and could also be suited to a wide variety of different situations.
2. Description of the Prior Art
For radio or television broadcasting antennas, there should be (barring exceptions) a radiating system having a diagram that is as omnidirectional as possible (the term "omnidirectional diagram" refers to a diagram that has no trough of less than 3 dB over 360 degrees).
This system should further have mechanical characteristics of compactness and lightness, enabling it to be placed at the top of a mast in minimizing both the static load (the inherent load of the radiating system) as well as the dynamic load (wind resistance) borne by the mast.
To this effect, it is most common to use an antenna system called a "panel antenna" formed with radiating elements, each consisting of a dipole placed before a reflector, the dipole being oriented vertically or horizontally depending on the desired polarization.
Since a radiating element such as this is an element with gain, hence a directional element, it has to be arranged in sets of four elements at angles of 90.degree. with respect to one another, to obtain the desired omnidirectional diagrams.
To increase the admissible power, a plurality of these radiating elements (most usually 2, 4 or 8 radiating elements) are superimposed so as to form radiating panels, the reflector most usually being common.
Each panel is powered separately with the same phase and the same power as all the others (unless it is sought to play on the form of the diagram in introducing phase shifts or power variations) by means of a distributor set.
Although this configuration works satisfactorily in a great number of present-day emitting stations, it has a number of drawbacks.
First of all, at the top of the mast itself, there should be provision for a small pylon enabling the appropriate arrangement of the superimposed radiating elements forming the four radiating faces of the configuration.
This element should meet two contradictory conditions:
firstly, its dimensions should be sufficient to enable the installation of supplies for each panel and enable a man to pass through the center of the configuration so that it is possible to provide for its maintenance. It has been noted, in effect, that each radiating element is supplied by its own coaxial cable and, since this coaxial cable necessarily has to be placed behind the reflecting panel of the radiating element so that it does not disturb its operation, the harness of coaxial cables will have to go through the inside of the small pylon which would therefore have to be high enough (for faces with 8 superimposed radiating elements, we thus have 32 coaxial cables to be inserted into this small pylon).
For this kind of ease of installation and also for sound mechanical rigidity, it is therefore desirable for the structure of the small pylon to be as wide as possible;
secondly, from the radio-electrical point of view, the troughs in the diagram get accentuated as and when the phase centers of the radiating elements get further away.
To obtain the most regular of diagrams possible, it is therefore desirable to bring the radiating elements of each group as close together as possible, and hence to provide for a small pylon section which is as small as possible (and at the same time restricted by the minimum dimension of the reflectors).
To reduce the above-mentioned static load and dynamic load to the minimum, it is also desirable to reduce the section of the small pylon to the minimum, all the more so as the radiating set should be shielded by a radome, the size of which, given the dimension of the radiating elements, will have a very large windward area and will therefore exert all the more force on the mast.
Another drawback of this type of antenna results from the complexity of its supply system (each of the radiating elements has to be supplied by its own coaxial cable as indicated above), and this makes it necessary to provide for a large number of secondary coaxial supply cables and connection boxes. The cost price of an antenna system such as this will thus increase very quickly with the number of radiating elements used.
Furthermore, the losses will increase very quickly, both because of the increase in the number of connection boxes and because of the lengthening of the secondary supply coaxial cables. Typically for a radiating system with a panel having eight superimposed radiating elements dimensioned for the 470-860 MHz band, the height of the small pylon and therefore, of the longest coaxial cables is of the order of 12 meters, thus creating considerable losses in a range of frequencies such as this.
Thus, typically, we arrive at small pylon sections of the order of 0.8.times.0.8 m. and radome diameters of the order of 1 meter for transmission in the 470-860 MHz band, one group of four panels with its radome having a mass of 350 to 400 kg. and having a windward surface of the order of 1.3 m.sup.2.
Another type of antenna suited to the above-mentioned use, although less used, is the so-called super turnstile antenna.
In this type of antenna, to obtain the desired omnidirectional diagram, the principle of the rotating field is used, and the radiating element then consists of two flat, vertical and mutually perpendicular "bat wings" intersecting at their center and mutually phase shifted by 90.degree..
Thus, a large number of radiating elements is superimposed, and each radiating element is supplied separately through a common distributor system and the two dipoles formed by the "bat wings" of each radiating element are supplied in quadrature non-periodically by a 3 Db coupler.
Although this type of antenna has a far smaller overall diameter than that of a system using antenna panels because of the absence of any reflector, it makes it possible to considerably reduce the dimensions of the small pylon, it has a certain number of drawbacks:
first of all, the need to achieve the non-periodic supply in quadrature between the dipoles causes the use of 3 dB couplers placed in the radiation field, the balancing charge of the coupler having to be dimensioned as a function of the power to be emitted;
then, the coaxial supply cables of the dipoles are located in the field of radiation of the antenna and, therefore, disturb its radiation by creating troughs in the diagram;
furthermore, for equal gain, the total height of the antenna is greater than that of an antenna with radiating elements, further causing correlative problems of compensation of the diagram in elevation for antennas with a large number of radiating elements;
finally, its cost price is very high because of the mechanical complexity, the presence of 3 dB couplers and the large number of coaxial supply cables used.
As can be seen, therefore, the two types of antennas used up till now for radio or television broadcasting emitters in the decimetric wavelengths are not entirely satisfactory because they cannot be used simultaneously to achieve both mechanical performances (compactness to restrict the windward surface, low weight and a structure that is easy to manufacture) as well as radio-electrical characteristics (omnidirectional nature of the diagram and the possibility of accepting high power) desired.